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A new study reports what might be the oldest fossil animals ever found. And we're also learning more about what role the hippocampus plays in certain types of visual memory.

Hosted by: Hank Green

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Sources:
https://www.nature.com/articles/s41586-021-03773-z
https://press.springernature.com/possible-poriferan-body-fossils-in-early-neoproterozoic-microbia/19347570
https://pubs.usgs.gov/fs/2007/3015/fs2007-3015.pdf
https://science.sciencemag.org/content/366/6471/1333.full

https://www.eurekalert.org/emb_releases/2021-07/piam-bc071921.php
http://dx.doi.org/10.1016/j.cub.2021.06.079

Images:
https://www.istockphoto.com/photo/seascape-of-coral-reef-in-the-caribbean-sea-around-curacao-with-various-corals-and-gm1042997770-279209291
https://www.istockphoto.com/photo/animals-of-the-cambrian-period-underwater-scene-with-anomalocaris-opabinia-gm1155964966-314896881
https://www.nature.com/articles/s41586-021-03773-z#Fig4
https://www.inaturalist.org/photos/136803694
https://www.istockphoto.com/photo/view-of-the-earth-from-space-gm482119728-69834959
https://commons.wikimedia.org/wiki/File:Pavilion_Lake_microbialite_towers.jpg
https://www.jpl.nasa.gov/images/lake-salda-rocks
https://www.istockphoto.com/vector/hippocampus-vector-illustration-labeled-diagram-with-isolated-closeup-gm1053259742-281397203
https://www.istockphoto.com/photo/stained-glass-seamless-texture-with-rectangle-pattern-for-window-colored-glass-3d-gm1191619458-338248677
https://www.istockphoto.com/photo/stained-glass-seamless-texture-colored-glass-with-rhombus-pattern-for-window-3d-gm1198679781-342668517
https://www.istockphoto.com/vector/how-eye-work-medical-illustration-eye-brain-diagram-eye-structure-and-connection-gm922038408-253156029
https://www.eurekalert.org/multimedia/530056
https://www.istockphoto.com/photo/wheres-waldo-gm95505710-11524580
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[♪ INTRO].

The early evolution of animals is shrouded in mystery, in part because the fossil record is simply spotty. But things might be a little bit clearer now, thanks to a new study in Nature that reports what might be the oldest fossil animals ever found.

Specifically, they are sponges. Sponges are among the most ancient animal groups, and models suggest they might have first evolved as early as one billion years ago. But the oldest definite fossils of sponges are only half that age.

They’re from the Cambrian period, a little over 500 million years ago. This lack of early fossils isn’t too surprising, considering that the earliest sponges were probably small, soft-bodied creatures that weren’t terribly abundant. All of those factors can lead to a sparse fossil record.

But now, through careful analysis of ancient reef remains from northwestern Canada, researchers think they might have found the fossils they’ve been hoping for. And the fossils are a whopping 890 million years old. The fossils in question are extremely small, tubular structures, only around 20 to 30 micrometers across, preserved among the carbonate minerals of the fossilized reef.

Structures like these are known to form when a specific type of sponges called horny sponges die and decay. Which, if you have a natural sponge in your shower... Yeah, it might be one of those.

If this identification is correct, it’s a big deal. At 890 million years old, these would be the oldest known animal fossils, and by far the oldest known sponges. Not only have we not found recognizable animal fossils from that time, we’re still trying to figure out whether animals could have evolved that early.

See, oxygen levels were much lower at that time. The so-called Neoproterozoic oxygenation event began around 800 million years ago, and saw a rise in oxygen levels in the atmosphere and the oceans. Scientists have wondered if that increase in oxygen was a requirement for the evolution of the earliest animals.

But since these new fossils seem to predate that event, that doesn’t seem to be the case. These sponges’ survival in those oxygen-poor times might have had to do with those reefs they were living in. These were not reefs full of coral and fish like we have today.

Those animals didn’t exist then. Instead, they were formed by photosynthesizing cyanobacteria, and photosynthesis releases oxygen. Those early sponges might have survived on the fumes of these reef-building microbes.

But at the same time, these early animals might have struggled to compete with their bacterial neighbors. These supposed sponge fossils are found in voids within the reef, places where cyanobacteria were rare and light levels were probably low. Long before animals grew to dominate such ecosystems, these earliest sponges may have scraped by in the shadows of early, microbe-dominated reefs.

If they’re sponges at all. Humble beginnings, but an exciting new clue to our own ancestral origins. So you’ve just seen a lot of visual information in a specific order, and if you remember it tomorrow, you might owe some thanks to a part of your brain called the hippocampus.

We already knew the hippocampus was involved in memory storage. However, new research in Current Biology reveals that it plays a role in certain types of visual memories, but not all. See, there are multiple ways for our brains to store memories of visual input.

In this study, the researchers examined two types of visual memory in mice. The first, called stimulus-selective response plasticity, is how both mice and humans learn to recognize single visual inputs. The mice were shown the same visual information over and over again, and as their brains stored it away, their electrical response became stronger and their behavior became less interested.

Which indicates they remembered it, and learned to ignore it. The second form of memory here is called visual sequence plasticity. This is how we learn to recognize and predict a series of images shown in a particular order.

When mice are shown a sequence over and over, their brains again react with stronger electrical signals, but the signal is weaker if the order of the images is changed. Their brains remember not only the images, but their sequence. Previous research has revealed that these memories are stored in a part of the brain called the visual cortex.

But the researchers wondered about the involvement of the hippocampus, which is known to be involved in some other forms of memory. To find out, they tested mice with damaged and undamaged hippocampus tissue. Both groups of mice did just fine with single-image memory, but the mice with damaged hippocampuses had no ability to remember a sequence of visual imagery.

Both their behavior and their brain’s electrical signalling showed that they simply weren’t remembering the sequences. This reinforces the idea that our brains can split up important tasks, like storing complex visual information. It also suggests visual memory doesn’t happen in one place, but is distributed across the brain.

And it could also lead us to better understanding memory disorders in humans, such as Alzheimer’s. More research in this area could answer some lingering questions, like precisely what the hippocampus is doing. There are a few caveats to this research.

For one thing, it was #inmice. And while the researchers studied what happened when the hippocampus is injured, that doesn’t necessarily tell us about what happens when it heals, or how the brain might adapt over time. But we know now which parts of the brain are involved, so we know where to look for more answers.

And that is information worth remembering. Meanwhile, we wanted to remember to thank our patrons, who make this and every episode of SciShow possible. If you’d like to get involved and join our amazing community, you can get started at patreon.com/scishow. [♪ OUTRO].